Most of us are interested in the corn plant as a whole but it is clearly an expression of its parts and those parts are an extension of the smallest parts, the cells. Cell function is dictated ultimately by components of the cell nucleus especially the DNA. The process of transforming a string of chemical compounds, the nucleic acids into ultimate structure and function of any living organism is amazing- or should we say: ‘a-maize-ing’.
DNA in each of the 10 chromosomes of each living cell of corn is composed 2 strands of DNA wound around each other. Each string is composed of nucleotides. Each nucleotide is composed of a sugar molecule of deoxyribose , a phosphoric acid molecule and a nitrogen molecule. There are 4 nucleotide molecule: Thymine, Cytosine, Adenine and Guanine. These are abbreviated as T,C,A and G. The sequence of these nucleotides in the DNA ultimately determine a gene and its product.
An enzyme causes the two strands of DNA to separate briefly to begin the RNA replica of a group of the DNA nucleotides. Some codes in the DNA called starter codes become the beginning of the RNA. The replication continues until it reaches another code called the stop code. This new RNA, strand migrates from the nucleus into the cell cytoplasm. It is called messenger RNA or mRNA as it is conveying genetic information to the ribosome in the cell.
The ribosome imports amino acids that are attached to each other according to the RNA nucleotide sequence. The string of different amino acids become a protein. The proteins enzymatic potential is determined by the sequence of the amino acids. This enzymatic power affects all other chemical processes needed for the living function.
We need to think both small and large about the corn plant.
Diversity among humans is obvious to us as our tendency is to look for physical features that are easily seen. But real diversity is hidden by those obvious features as internal differences and culture are the real diversity. Corn diversity affected by mutations in DNA and RNA for multiple differences in adaptation to environments and the balance we demand between for grain production, quality and harvestability. Some are obvious but much goes unseen.
Not only are small changes in ‘error’ in duplication of chromosomal DNA significant but RNA, the chain of nucleic acids transferring the codes from the chromosomes to the ribosomes for protein construction, can have their own errors. In both cases, proteins essential for some physiological process can be affected. Transportation of glucose to roots, production of new cells or number of stomates can be affected, causing drastic affects on final performance of the corn plant.
Production of the components that allow the recognition of microbe-associated molecular patterns is an example of an essential physiological component to the plant being able to respond to a pathogen attack. Critical mutations in production of this system are an import component to resistance systems.
Corn’s exposure to multiple environments allows us to discard those with detrimental mutants, accounting for the relatively short life of any commercial hybrids. Fortunately, the long, varied history of this annual crop has allowed for a vast genetic base to draw upon for new genetic combinations, and mutations, to draw upon for final performance in expected environments of the next season.
Just as in humans, some of those obvious, visible trait difference do not predict the inner differences. It is performance that is importance.
How diverse is corn? That issue is often expressed with a concern that it is becoming too narrow. Certainly, the selection pressure for performance under today’s USA agriculture environment does move the genetics towards performance in environments that have changed during the past 40 years. Higher plant density and more minimum tillage have increased needs for more tolerance of stresses on plants. Other plant characteristics have also been chosen in today’s commercial needs for grain quality.
But is more diversity, if needed, available? We tend to only recall the diversity in the characteristics that we see or receives our attention. If corn is viewed from the road as we pass by fields, it looks the same in nearly every field. If one is a student of corn, one sees a range of leaf structures, kernel depths, kernel quality, root structure, flowering timing, and tassel branches. Measuring grain and standability differences at the end of the season shows diversity at the end of the season. These observations of outward characteristics are not a complete analysis of the unseen diversity that may or may not be expressed- at least to us.
Mutations that occur with every reproduction often do not affect physiological processes that we observe. Some may affect some process that has no affect in current environment but may be significant later. Maize chlorotic mottle virus became significant in Nebraska in 1976. Although most common hybrids were susceptible, a few older inbreds were found to be resistance. When the disease broke out in Africa, within a few years breeding programs identified genotypes with resistance. Goss wilt, caused by a bacterium that apparently came from grasses, caused severe damage to a few popular corn genotypes, but resistance was found in other adapted corn hybrids. Unhidden diversity within corn has continually contributed to undesirable characteristics, such as susceptibility to a ‘new’ disease and also to resistance to a potential pathogen.
Corn’s history of movement to multiple environments, its annual reproduction and large number of genes have contributed to an immense diversity that is available for future versions of the crop.
Many mutations occur during cellular replication but those occurring in haploid cells can have extreme expression because often these are in recessive genes. Dominant versions of the mutated, recessive gene are covered up in most diploid genotypes. Inbreeding objective is to make all genes homozygous as the breeder attempts to obtain consistent, repeatable genetics but with the potential cost of making homozygous some recessive genes with negative effects on the plant. Not only does this result in smaller corn plants as the inbreeding progresses, but also carries risk for a few diseases.
One example is susceptibility to Race 1 of Bipolaris zeicola (Helminthosporium carbonum). This variant of the fungus apparently is among other grass leaf pathogens of this species. It has genetics resulting in production of a toxin that is controlled by a dominant gene in corn. During the inbreeding process, however, and recessive version of this gene is made homozygous. Consequently, these inbreds are frequently heavy infected in many seed fields exposed to the pathogen. More information on this pathogen race can be found in 7/11/19 blog of Corn Journal.
Much of the increase in corn grain production, adaptation to multiple environments, disease resistance (and susceptibility), and specialty traits are the result of naturally occurring genetic mutations in this annual plant. Humans benefit that mutations occur in corn, as other forms of life, but we should not be surprised with changes from mutations that are often expressed in inbreds- and hope the other parent of a commercial hybrid covers up the defects.
Corn, being an annual plant, has opportunity for quick expression of mutations. Simple substitution for one of the nucleotide bases within the DNA for a gene, can result in an amino acid change in a protein critical to some physiological process in the plant. Many of these types of ‘errors’ made during meiosis do not cause meaningful or notable changes to the expressed corn phenotype, but a few have huge effects.
Opportunity for mutations to occur among the 40,000 corn genes, each gene having about 50-150 sets of 3 nucleotide bases makes genetic variability inevitable. Fortunately for us, this variability has been mostly beneficial, allowing for adaptation of this crop to multiple environments. Humans have been able to select those appropriate genotypes. Some rare mutants resulting in waxy and high amylose endosperm have special uses. White endosperm is another recessive gene that was due to a mutation in gene responsible for carotenoids in yellow kernels. Pure white kernels also require genes for colorless pericarp. Other obvious mutations involve different anthocyanin genes resulting in blue and red kernels.
Most mutations have resulted in less obvious differences affecting plant height, leaf uprightness, leaf width, root growth and direction, mineral uptake, general and specific disease resistance and even more subtle differences in photosynthesis and cellular respiration.
Mutations, although random, have given us a crop adapted to multiple human needs.
A review of the relationship of nucleotides to DNA and RNA can be found at:
A maize data base of corn mutants is maintained at maizegdb.org. This online site has an interesting summary of notable mutants at:
Not only is nuclear DNA vulnerable to mutations but also that in mitochondria. One of the dramatic examples was the occurrence of sterile cytoplasm in corn. And the unexpected variant in a pathogen that attacked the variant mitochondria. Below is a post from 7/27/2017.
MITOCHONDRIAL DNA, CORN MALE STERILITY AND RACE T
All living cells of plants and animals have mitochondria, organelles that convert carbohydrates into the useful form of energy that drives synthesis of metabolites in cells. Mitochondria are believed to be descendants of bacteria that became symbiotic with cells in the early evolution of most living forms. They retained their own DNA, are transferred to the next generation only in eggs cell and not sperm. They replicate within cells but the host cells have some control on the rate of replication. Energy conversion in mitochondria occurs on their folded membranes in a series of chemical reactions. Regions of the plant undergoing rapid cell duplication have more mitochondria. This includes the tassel cells of a corn plant. The pollen mother cells in that region undergo meiosis and duplication, driven partly by the energy conversion by concentration of mitochondria in those mother cells.
A small defect in mitochondrial DNA of an inbred caused a defective membrane product in those mitochondria resulting in incomplete development of pollen. This was found in a corn breeding program in Texas. As the inheritance of this condition was known to be only transmitted independent of nuclear DNA, it was called Texas male sterile cytoplasm. It became a useful tool to corn hybrid seed production because it was easily transferred in breeding programs to the female parent of a hybrid, and thus avoiding manual removal of tassels in seed production fields. Use of T male-sterile cytoplasm became common in the worldwide corn in the 1960’s.
It was noted in the Philippines in 1961, that a fungal pathogen, then known as Helminthosporium maydis, was especially aggressive on several hybrids with T cytoplasm. Despite a few scattered reports elsewhere it was not until 1969 that the connection between increased occurrence of this disease and T cytoplasm became alarming. Majority of seed produced for 1970 corn season had T cytoplasm, the main exceptions being new hybrids in which the conversion to sterility of the female parents was incomplete.
Although the pathogen was normally found in the southern half of the corn belt, and adequately controlled by products of nuclear DNA genes, this disease was found highly destructive in northern corn belt areas as well. A race of the fungus (now named Bipolaris maydis and by its sexual stage Cochliobolus heterostrophus) called race T, produces a toxin that causes death to cells with mitochondria having the DNA with the defect associated with T male sterility. All cells of the corn plant with these defective mitochondria were vulnerable to the fungus. This included the cells in developing seed resulting in diseased stored grain as well as overwintering leaves and stalks. Normal resistance mechanisms to the pathogen were ineffective because the toxin destroyed these defective mitochondria.
As the relationship with T cytoplasm was realized, seed companies worked to change, and within a few years, the disease subsided back to its normal distribution. It was a new learning experience of interaction of corn and pathogen biology.
Corn’s past and its future is driven by mutations, allowing for humans to select desirable characteristics. Because most meaningful mutations generational result in changes is recessive genes that only become expressed when, in diploids, homozygous for that recessive gene, the mutations may not become evident in hybrids but only after inbreeding.
Bacteria and fungi also have mutations but most of their life cycle is controlled by haploid versions of DNA. Consequently, a mutation can have immediate effect on a potential pathogen. Most of these organisms have high rates of reproduction and spread. Mutations in a potential pathogen resulting in a new protein that allows blocking the detection of a pathogen by the host plant can result in success of the pathogen to further invade the plant. Races of Exserohilum turcicum have specific mutations that block the turning on of specific lesion size restriction in corn of Ht1, Ht2, Ht3 and Ht4. Clavibacter michiganensis is a bacterium species with multiple subspecies that are essentially mutants adapted to specific hosts including wheat and tomatoes. A mutant was identified on corn in Nebraska in 1970 as the cause of Goss’s wilt of corn.
Mutations in potential pathogens will continue, as will mutations in corn. Diversity is good, most of the time, and necessary for all of us into the future.
We can be thankful for naturally occurring mutations. It is basic to providing the eventual variability that has driven and continually drives evolution. It allowed the deviants in Teosinte that was selected by people in Mexico 10000 years ago and the multiple selections in corn as it was moved worldwide since then. Most research has verified that most of these genetic mutations result in recessive genes and thus the presence of the mutation is not often expressed in a diploid present in which the dominant member of the paired gene is expressed. The su genes resulting in sweet corn is only expressed when the recessive gene is expressed in both members of the diploid plant. Same is true of the mutants wx for waxy. This is true for multiple other homozygous recessive traits.
Occurrence of mutations can be an advantage or a disadvantage. In most cases, being recessive, the mutation may not be detected by performance of the hybrid. Selfing to achieve homozygosity during the inbred development reflects the negative affect of making some recessive genes more homozygous. This is reflected in reduction of plant size from the heterozygous parent used for inbred development. The selection process with each generation does allow elimination of some negative homozygous recessives. Double haploid systems do not allow generational selection because the homozygous condition is fixed.
Expression of hybrid vigor when an inbred is crossed with another specific inbred is mostly due to dominant versions of the negative recessive genes of the inbred parents. That is probably why prospective commercial hybrids are from crosses of inbreds with distinct ‘families’, each not likely to share the same negative recessive versions of important genes. Corn has 40000 genes, including some negative recessives, perhaps due to mutations. The seed industry uses hybrid testing, and inbred development to select for hybrid performance. Further selection among those near-inbreds can allow for selection against the few negative traits found among some plants to improve inbred performance in hybrid production. That has been consistent with our experience in our proprietary Rapid Inbreeding® program. Diversity is good.
Differences among all living things is driven by mutations. A simple change in the nucleic acid position within the DNA or RNA code can affect the structure of the protein being produced by a gene. That protein’s function among the several affecting production of some cellular process ultimately can be significant to the organism’s appearance or function. Every cell division is vulnerable to such slight changes in these types of slight changes in DNA. Those occurring during cell division of cells not involved in reproduction result in a group of cells differing from adjacent cells is called a chimera. It could result in a distinct streak in a leaf of one corn plant. Most chimeras do not continue to the next generation if the mutation was not present in the nuclei of the reproduction cells.
Mutations during meiosis are the main source of genetic diversity in organisms with two sets of chromosomes (diploids). Multiple mutations accumulate over generations resulting in distinctive characteristics among genotypes. Organisms with shorter life cycles are likely to gain diversity quicker. Selection of those best fitting an environment can include variations utilizing different physiological processes to accomplish this success. The randomness involved in mutations also includes differences that we humans do not know are of necessary function but simply are present.
These genetic differences among corn varieties can be detected by comparing plant structures, proteins or DNA. Professional Seed Research, Inc. compares leaf structures of three leaf plants when growing in uniform environments, allowing the distinction between expected phenotype versus those produced by the wrong parent, such as selfing or pollen from wrong male parent. Careful observation of mature plants also makes this distinction. In all of these methods, we detect diversity but not necessarily significant differences in desired function of the plant.
Although diversity is frequently random or at least without recognized useful function, we use it as a means to detect distinct varieties. Taxonomists specialize in observations of these differences to define species. Humans use phenotype expression of facial characters to distinguish among individuals. It is best, at this point, not to comment about those human characteristics most desirable to me, but just as with corn, some diversity due to genetic diversity would seem less functional than others.
Among the 30000-40000 genes in corn, some mutations have occurred over time to be useful to humans and some are distinctive but not obviously useful in today’s environments. In general, we should celebrate diversity in all organisms. Mutations are great!
About Corn Journal
The purpose of this blog is to share perspectives of the biology of corn, its seed and diseases in a mix of technical and not so technical terms with all who are interested in this major crop. With more technical references to any of the topics easily available on the web with a search of key words, the blog will rarely cite references but will attempt to be accurate. Comments are welcome but will be screened before publishing. Comments and questions directed to the author by emails are encouraged.